Endoscopes are used to view cavities in the human or animal body and also in technical objects. An endoscope typically comprises an elongate endoscope shaft, which is suitable for insertion into the cavity to be viewed, and a head that can have attachments and control elements and also an eyepiece lens. Inside the endoscope shaft and the head there is an optical system for transmitting an endoscopic image from the distal end of the endoscope, i.e. the end remote from the observer, to the proximal end, i.e. the end near the observer. For this purpose, the optical system comprises in particular a distally arranged objective for receiving the endoscopic image, an image carrier, and an eyepiece arranged at the proximal end of the endoscope, with an eyepiece lens for viewing the transmitted endoscopic image. The endoscope can be designed in particular as a rigid endoscope with a rigid endoscope shaft and can have, as its image carrier, an arrangement of rod lenses accommodated in an optics tube arranged inside the endoscope. Moreover, the endoscope can have an illuminating device for illuminating the cavity to be viewed, and, if appropriate, it can have further devices, such as channels for endoscopic work instruments.
To assemble the optical components in the optics tube of a rigid endoscope, they are usually pushed into the optics tube from the proximal end and in the sequence and arrangement predefined by the lens design. In particular, the objective, one or more rod lenses and one or more aperture stops or spacers are pushed in succession into the optics tube. For this purpose, the optical components have an external diameter that is slightly smaller than the internal diameter of the optics tube. With the aid of a system spring and a system closure guide, the entire optical system is pretensioned in the distal direction and thereby pretensioned against a distal closure piece of the optics tube, which can be provided in particular by a cover glass. In this way, the optical elements are held in the longitudinal direction in the respective position that has been predetermined on the basis of the lens design.
During their use, endoscopes are exposed to considerable mechanical and thermal stresses. Thus, during their handling, the endoscope can be subjected to accelerations and vibrations or jolts, or the endoscope shaft may be bent if handled without due care. Accelerations and in particular vibrations can lead to movements of the lenses and of the other optical components relative to one another and relative to the optics tube, possibly resulting in abrasion, which leads to the development of stray light. Moreover, the optical elements may tilt relative to one another and/or move axially out of place relative to one another, which can likewise impair the quality of the endoscopic image conveyed. Vibrations and bending of the endoscope shaft can cause the lenses to bulge or even break, the rod lenses being particularly at risk on account of their length. Moreover, medical endoscopes are exposed to hot steam and high pressure when sterilized by autoclaving, in which case leaks in the seal of the optical system can likewise lead to an impairment of the endoscopic image. Considerable costs can thus arise in the production and in the use of an endoscope.
In order to fix the rod lenses safely and in the correct position in the inside of the optics tube, and to avoid bending of the endoscope shaft being transferred to the rod lenses, it is known from the laid-open application DE 19750685 A1 that the rod lenses are positioned at a defined radial distance from the inside face of the optics tube by means of at least two centering aids, and that an elastic adhesive connection is produced between the outside of the rod lens and the inside face of the tube at at least two locations spaced axially apart from each other. Each centering aid has centering elements distributed uniformly about the circumference of the rod lens.